Currently, the new cellular mobile communication standard LTE (long term evolution) is being implemented globally. As one main new feature, this standard comprises the possibility to use two receive channels at the same time at different frequency bands to improve the downlink data rata. According to the newest releases to this standard, a series of inter-band combinations are defined each comprising two Rx frequency bands in which receive signals have to be operated at the same time by a mobile phone. In the LTE standard, the combination of different frequency bands, in the following called band aggregation, concerns up to now FDD (Frequency Division Duplexing) frequency bands only comprising different frequency bands for transmit and receive channels. About 30 FDD frequency bands are yet defined by the mobile community but few of them are proposed for band aggregation operation mode.
In an FDD frequency band, simultaneous transmit and receive operation is possible. Thus, each FDD frequency band needs a duplexer and a standard like LTE needs two duplexers to be connected to the antenna at the same time to support interband carrier aggregation. Up to now, no technical solution is known how to construct a front-end circuit with two duplexers that can be operated without any performance degradation or without any additional requirements on the duplexers. This is due to the simultaneous matching of the two duplexers to each other, to a switch and to the antenna feed. Currently used front-end circuits use a single-feed antenna followed by a multi-throw switch that connects one duplexer at a time to the antenna. For such a front-end, the support of band aggregation would mean that the switch needs to be able to have two active paths according to proposed and future band aggregation modes. Besides an enabling of the switch for this new operation mode, more importantly, the duplexer pair that has to be connected to the same antenna port simultaneously needs to be matched so that the duplexers do not load each other. Such a matching unavoidably incurs losses at the front-end but also requires additional area on the front-end. If the existing front-end already supports several band combinations, this would make the design challenging and prone to yield loss. Secondly, the single duplexers need to have an extremely good out-of-band reflectivity so that the duplexers are not loading each other, which otherwise would increase RF front-end losses significantly. Further, as known front-ends use different filter technologies in one module, these losses cannot be compensated within a module design. Even lossless matching components cannot solve this task if using a known architecture of the front-end.